Boiling water nuclear reactors contain discrete fuel bundles clustered together in the central portion of the reactor to form the steam generating core of the reactors. These fuel bundles have lower tie plates for supporting a group of upright fuel rods and admitting water moderator from the lower regions of the nuclear reactor. The bundles include an upper tie plate for maintaining the fuel rods upright and permitting water and generated steam to exit upwardly from the fuel bundle to the upper regions of the nuclear reactor. A channel surrounds both tie plates and the fuel rods extending therebetween to confine the flow path of the moderator between the tie plates and around the steam generating fuel rods. Additionally, fuel bundles contain fuel rod spacers distributed at vertical intervals from the bottom of the fuel bundle to the top of the fuel bundle. These spacers have the mechanical function maintaining the matrix of fuel rods in precise side-by-side relation. This prevents the otherwise flexible fuel rods from coming into abrading contact under the forces of the upward hydraulic flow as well as maintaining the fuel rods in their designed side-by-side relation for improved nuclear performance.
Operation of the fuel bundles within the reactor can be described in terms of both thermal hydraulic performance and nuclear performance.
In terms of thermal hydraulic performance, moderator in the liquid state enters the bottom of each fuel bundle through the lower tie plate, and flows upwardly within the channel and between the fuel rods. During this upward flow increasing amounts of vapor (steam) are generated. At first and in the lower portion of the fuel bundle, liquid flow predominates with an upwardly increasing array of vapor bubbles. Later and in the upper extremities of the bundle, vapor flow predominates with liquid forming an increasingly reduced fraction of the upward moderator flow. It is known that the vapor (steam) fraction of the upwardly flowing steam--water mixture is lowest adjacent the channel or sides of the fuel bundle and highest in the center of the fuel bundle. Therefore, the central upper portion of a fuel bundle in a boiling water reactor will have a high void fraction (a predominance of vapor over water). This high void fraction effects the nuclear performance of the fuel bundle.
Regarding reactor nuclear performance, in a boiling water nuclear reactor, the density of the water is important. Simply stated, the nuclear reaction generates fast neutrons. The continuance of the nuclear reaction requires slow or thermalized neutrons. It is the function of the moderator to moderate the fast neutrons to the thermal state so that the reaction can continue.
The sufficiency of this moderation is a function of the density of the moderator at any particular point within the interior of the reactor. As has already been mentioned, moderator density in the central upper region of the fuel bundle is low.
To correct this condition, it is well known to insert so-called water rods in the interior of a boiling water nuclear reactor fuel bundle. These water rods are filled with liquid moderator to supply to the upper region of the fuel bundle the necessary moderator density for the efficient nuclear reaction.
It is to be understood that conventional water rods, while having the nuclear efficiency of supplying water moderator to the upper central portion of the fuel bundle, have a thermal hydraulic deficiency. Specifically, and in order that the water rods remain full with liquid moderator, water is taken from the bottom of the fuel bundle and shunted directly to the top of the fuel bundle. The water bypasses otherwise bypasses the steam generating flow within the fuel bundle.
It is the purpose of this invention to provide a fuel bundle with an improved water rod that both continues the water density in the upper two phase region of fuel bundles having water rods and yet improves the thermal hydraulic characteristics of the fuel bundle.